一、备忘录模式介绍
1、定义与类型
定义：保存一个对象的某个状态，以便在适当的时候恢复对象。
"后悔药"
类型：行为型
2、适用场景
保存及恢复数据相关业务场景
后悔的时候，即想恢复到之前的状态
3、优点
为用户提供一种可恢复机制
存档信息的封装
4、缺点
资源占用
5、相关设计模式
备忘录模式和状态模式
二、代码示例
模拟场景：在网站上发布手记，暂存手记的不同版本
手记类：
public class Article { private String title; private String content; private String imgs; public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } public String getContent() { return content; } public void setContent(String content) { this.content = content; } public String getImgs() { return imgs; } public void setImgs(String imgs) { this.imgs = imgs; } public Article(String title, String content, String imgs) { this.title = title; this.content = content; this.imgs = imgs; } public ArticleMemento saveToMemento(){ ArticleMemento articleMemento = new ArticleMemento(this); return articleMemento; } public void undoFromMemento(ArticleMemento articleMemento){ this.title = articleMemento.getTitle(); this.content = articleMemento.getContent(); this.imgs = articleMemento.getImgs(); } @Override public String toString() { return "Article{" + "title='" + title + '\'' + ", content='" + content + '\'' + ", imgs='" + imgs + '\'' + '}'; }}存档类：
public class ArticleMemento { private String title; private String content; private String imgs; public ArticleMemento(Article article) { this.title = article.getTitle(); this.content = article.getContent(); this.imgs = article.getImgs(); } public ArticleMemento(String title, String content, String imgs) { this.title = title; this.content = content; this.imgs = imgs; } public String getTitle() { return title; } public String getContent() { return content; } public String getImgs() { return imgs; } @Override public String toString() { return "ArticleMemento{" + "title='" + title + '\'' + ", content='" + content + '\'' + ", imgs='" + imgs + '\'' + '}'; }}存档管理类：
public class ArticleMementorManager { private final Stack<ArticleMemento> ARTICLE_MEMENTO_STACK = new Stack<ArticleMemento>(); public ArticleMemento getMemento(){ ArticleMemento articleMemento = ARTICLE_MEMENTO_STACK.pop(); return articleMemento; } public void addMemento(ArticleMemento articleMemento){ ARTICLE_MEMENTO_STACK.push(articleMemento); }}测试类：
public class Test { public static void main(String[] args) { ArticleMementorManager articleMementorManager = new ArticleMementorManager(); Article article = new Article("如影随形的手记A", "内容A", "图片A"); ArticleMemento articleMemento = article.saveToMemento(); articleMementorManager.addMemento(articleMemento); System.out.println("标题：" + article.getTitle() + ",内容：" + article.getContent() + ",图片：" + article.getImgs()); System.out.println("完整笔记信息：" + article); System.out.println("-分割线------------------------------------"); System.out.println("修改手记start"); article.setTitle("如影随形的手记B"); article.setContent("内容B"); article.setImgs("图片B"); System.out.println("修改手记end"); articleMemento = article.saveToMemento(); articleMementorManager.addMemento(articleMemento); System.out.println("标题：" + article.getTitle() + ",内容：" + article.getContent() + ",图片：" + article.getImgs()); System.out.println("完整笔记信息：" + article); System.out.println("-分割线------------------------------------"); article.setTitle("如影随形的手记C"); article.setContent("内容C"); article.setImgs("图片C"); System.out.println("暂存回退start"); System.out.println("回退出栈1次"); articleMemento = articleMementorManager.getMemento(); article.undoFromMemento(articleMemento); System.out.println("完整笔记信息：" + article); System.out.println("回退出栈2次"); articleMemento = articleMementorManager.getMemento(); article.undoFromMemento(articleMemento); System.out.println("暂存回退end"); System.out.println("完整笔记信息：" + article); }}输出：
标题：如影随形的手记A,内容：内容A,图片：图片A
完整笔记信息：Article{title='如影随形的手记A', content='内容A', imgs='图片A'}
-分割线------------------------------------
修改手记start
修改手记end
标题：如影随形的手记B,内容：内容B,图片：图片B
完整笔记信息：Article{title='如影随形的手记B', content='内容B', imgs='图片B'}
-分割线------------------------------------
暂存回退start
回退出栈1次
完整笔记信息：Article{title='如影随形的手记B', content='内容B', imgs='图片B'}
回退出栈2次
暂存回退end
完整笔记信息：Article{title='如影随形的手记A', content='内容A', imgs='图片A'}
三、源码示例
1、spring中的webflow
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